Auxins

Introduction: what is growth?

Before we talk about auxins, let us agree on what we mean by "growth." In plant physiology, growth is understood as an irreversible increase in the size and mass of the organism, which occurs through the formation of new structural elements—cells, tissues, and organs (Kuznetsov, 2006). This is not merely an increase in volume (e.g., seed imbibition in water) but a process of de novo structure formation. This definition, given by the outstanding Russian physiologist D. A. Sabinin, emphasises a crucial feature of plants: they grow throughout their entire life, continually creating new organs and tissues.

But a fundamental question arises: how does a plant, being a sessile organism, create its spatial organisation? How does it "know" where the root should be and where the shoot should be? Why does the root grow downwards and the shoot upwards? Why do branches have a certain angle of divergence from the trunk? The answer to these questions is inextricably linked to auxins—the first and perhaps the most important group of plant hormones (Hopkins & Hüner, 2009; Schopfer & Brennicke, 2016).

Discovery of auxin: how our understanding began

The story of auxin’s discovery is truly instructive. In 1880, Charles Darwin and his son Francis conducted experiments with canary grass seedlings (Phalaris canariensis). They noticed that if a seedling was illuminated from one side, after some time it bent towards the light source. However, the most interesting finding came when they removed or covered the tip of the seedling—the coleoptile—with a cap. It turned out that light perception occurs precisely at the tip, while the bending occurs in the growth zone below (Schopfer & Brennicke, 2016; Hopkins & Hüner, 2009).

The Darwins made a brilliant inference: the tip transmits "some influence" to the lower part, causing it to bend. Subsequent work by Boysen-Jensen (1913) and Paál (1918) showed that this influence could pass through an agar block and therefore had a chemical nature. However, it was not until 1926 that the Dutch scientist F. Went succeeded in isolating this substance. He developed a brilliantly simple method: he placed excised coleoptile tips on agar blocks, and then applied these blocks (now containing the diffused substance) asymmetrically to decapitated coleoptiles. The coleoptiles bent away from the block, and the angle of curvature was proportional to the amount of active substance (Hopkins & Hüner, 2009; Schopfer & Brennicke, 2016).

Went named this substance auxin (from the Greek auxein – to increase, to grow). Later (1934–1946), it was established that the natural auxin is indole-3-acetic acid (IAA) (Hopkins & Hüner, 2009; Medvedev, 2012).

Why is auxin "the main" one? The uniqueness of auxin among phytohormones

Plants possess several groups of hormones—auxins, gibberellins, cytokinins, abscisic acid, ethylene, brassinosteroids, and others (Medvedev, 2012; Schopfer & Brennicke, 2016). Each performs important functions. Why then is auxin given a special role? The reason lies in the unique properties of auxin, which are found in no other phytohormone:

1. Auxin is an "instruction," not just a "fertiliser." Unlike nutrients that simply provide energy and building materials, auxin carries information about how and in what direction an organ should grow. It does not merely accelerate growth but organises it in a specific manner.

2. Auxin operates through concentration gradients. The biological effect of auxin depends on its concentration in the tissue. Moreover, the same concentration may be optimal for one organ and inhibitory for another. For instance, a concentration of auxin that stimulates shoot growth strongly inhibits root growth (Hopkins & Hüner, 2009; Medvedev, 2012). This property underlies the remarkable ability of one hormone to elicit different responses in different parts of the plant.

3. Auxin determines position in space. It is the directed redistribution of auxin in response to external stimuli (light, gravity) that causes tropisms—growth movements that ensure the spatial orientation of plant organs (Kuznetsov, 2006; Medvedev, 2012).

4. Auxin is a hierarchy-creating hormone. Through polar transport (movement in only one direction), auxin forms concentration gradients in the plant, determining which bud will be the main (apical) one and which will be lateral and subordinate (Medvedev, 2012; Schopfer & Brennicke, 2016).

It is these unique properties that make auxin the primary coordinator of the spatial organisation of the plant body.

Auxin and the concept of gradient: from "here" to "there"

Imagine the plant as a city. For an ordered structure to emerge, an addressing system is needed. Auxin creates such a system by forming concentration gradients along the axes of organs. A cell "knows" where it is by the amount of auxin surrounding it. The closer to the shoot tip, the higher the auxin concentration; the farther away, the lower. This continuous gradient serves as the very "instruction" that determines which processes will occur in each particular cell (Connor et al., 2011; Schopfer & Brennicke, 2016).

One could say that auxin is not just a hormone but a morphogen: a substance that, depending on its concentration, triggers different developmental pathways in cells. At high concentrations, it stimulates division and elongation of shoot cells; at intermediate concentrations, it promotes the formation of lateral roots; at low concentrations, it stimulates their growth. This gradient model allows the same substance to govern the most complex architecture of the plant (Hopkins & Hüner, 2009; Medvedev, 2012).

Main takeaway of this section:

Auxin is the main growth hormone not because it is the most potent, but because it creates the spatial organisation of the plant. It carries the information about where the top is and where the bottom is, which organs should be dominant and which subordinate. It is not merely a growth stimulator but a conductor, directing the entire orchestra of development. In the following sections, we will see how this remarkable mechanism works in practice.

Polar transport—a unique property of auxin

If you ask a plant physiologist, "What is the main difference between auxin and all other hormones?" the answer will almost certainly be: polar transport. This property makes auxin unique among phytohormones. Gibberellins, cytokinins, and abscisic acid can move through the plant in various directions—via xylem, phloem, with water and assimilate flows. Auxin, however, moves in a strictly defined manner: from the shoot tip to its base, from young leaves to the roots. This directed, vectorial movement is called polar transport (Medvedev, 2012; Hopkins & Hüner, 2009; Schopfer & Brennicke, 2016).

What does this mean in practice? If we take a segment of a stem or coleoptile and place radioactive auxin on one end, after some time we will find it at the opposite end—but only if that end is the morphologically basal one (i.e., oriented towards the root). If we turn the segment over, the auxin will still move from the end that was originally the top to the end that was the bottom. The direction of transport is determined not by gravity but by the internal polarity of the tissue (Schopfer & Brennicke, 2016; Taiz et al., 2023). This is a fundamental property, established during embryogenesis and maintained throughout the plant’s life.

Why does this matter? The gradient as a basis for organisation

Imagine the plant as a city with one-way streets. Auxin is a vehicle that can only travel from the centre (the shoot tip) to the periphery. This creates a stable concentration gradient: auxin is abundant at the shoot tip and becomes increasingly scarce towards the roots. This gradient serves as the addressing system we discussed in the first section (Connor et al., 2011; Schopfer & Brennicke, 2016).

Thanks to polar transport, auxin fulfils several critical functions:

• Establishes the main axis of the plant (apical-basal polarity).
• Creates an organ hierarchy: the closer to the tip, the higher the auxin concentration and the stronger the suppression of lateral buds (apical dominance).
• Enables directed responses to external stimuli: under unilateral illumination or changes in orientation, auxin is redistributed, creating an asymmetry that leads to bending (tropisms) (Kuznetsov, 2006; Medvedev, 2012).

Without polar transport, the plant could not build its architecture. This is why it is said that polar transport is the "heart" of the auxin control system.

How polar transport works: the cellular level

To understand the mechanism of polar transport, imagine an individual cell in a vascular bundle (e.g., in a stem or coleoptile). Auxin (IAA) is a weak organic acid (pKa ≈ 4.75). This means that in an acidic environment (e.g., in the cell wall, pH about 5.0–5.5), some molecules exist in the undissociated, lipophilic form (IAAH), which can easily cross the membrane. In the cytoplasm, where pH is about 7.0, most molecules dissociate into the anion IAA⁻, which cannot freely pass back through the membrane (Hopkins & Hüner, 2009; Medvedev, 2012; Schopfer & Brennicke, 2016).

This creates an "ion trap": auxin enters the cell easily (through any side), but can exit only through specialised carrier proteins (efflux transporters) that are located on a specific side of the cell—its basal (root-facing) pole (Medvedev, 2012; Taiz et al., 2023). This asymmetric localisation of efflux proteins is the key to polarity.

Now imagine a chain of cells: each one "ejects" auxin through its basal end, and it enters the next cell, where the process repeats. This creates a continuous flow from the apical (upper) end of the tissue to the basal (lower) end. The speed of this movement is about 5–20 mm per hour—much faster than simple diffusion, but slower than phloem transport (Medvedev, 2012; Schopfer & Brennicke, 2016). This indicates that the transport is active and energy-dependent—it is inhibited by respiratory inhibitors and under anoxic conditions.

Molecular "drivers": PIN proteins and other carriers

The main players in polar auxin transport are proteins of the PIN family (from pin-formed). They got their name from the phenotype of pin1 mutants in Arabidopsis: such plants develop a "pin" without flowers instead of a normal inflorescence because auxin transport and consequently organ initiation are disrupted (Medvedev, 2012; Taiz et al., 2023).

PIN proteins are efflux carriers that are localised polarly in the plasma membrane—on the basal (for PIN1) or apical (for PIN2) side of the cell. It is their asymmetric localisation that determines the direction of auxin transport through the tissue (Medvedev, 2012; Schopfer & Brennicke, 2016). How is this polar localisation achieved? It is regulated by vesicular trafficking: PIN proteins are continually transported from the endoplasmic reticulum to the plasma membrane and then to specific domains with the help of the actin cytoskeleton and accessory proteins (e.g., ARF/GEF factors). Mutations in these regulatory systems (e.g., gnom) lead to a complete loss of polarity and embryonic lethality (Medvedev, 2012).

In addition to PIN proteins, ABC transporters (PGP-type phosphoglycoproteins) also participate in transport—they provide energy-dependent auxin efflux, working in synergy with PIN proteins. For auxin entry into the cell, AUX1/LAX proteins, which are influx carriers and are more evenly distributed in the membrane, serve this function (Medvedev, 2012; Taiz et al., 2023; Hopkins & Hüner, 2009).

In summary, polar transport consists of three steps:

1. Entry of auxin into the cell (passively through the membrane or via AUX1).

2. Ion trapping – dissociation in the cytoplasm.

3. Exit via PIN proteins (and ABC transporters) at the basal end of the cell.

Polar transport and phloem: two different pathways

It is important to emphasise that polar transport is not the only way auxin moves. Most auxin synthesised in mature leaves is transported via the phloem together with sucrose, and this movement is not polar—it is driven by pressure gradients and directed towards "sinks" (actively growing organs). The speed of phloem transport (5–20 cm/h) is significantly higher than that of polar transport (0.5–1.5 cm/h) (Medvedev, 2012; Marschner, 2012).

These two pathways serve different purposes. Phloem transport provides a "baseline" level of auxin in tissues, while polar transport is used for fine-tuning, creating local gradients and directing growth. They operate in parallel and independently, but in certain situations they may exchange auxin (though the mechanisms of this exchange are not yet fully understood) (Medvedev, 2012).

What does polar transport give the plant?

Let us return to the key question: how does the plant determine the direction of its growth? Polar transport creates an axial gradient of auxin, and this gradient serves as the physiological basis for:

• Formation of the main axis – the direction from tip to root is established.
• Establishment of apical dominance – the shoot tip receives the highest amount of auxin, which inhibits the development of lateral buds.
• Tropisms – when external conditions change (light, gravity), auxin is redistributed asymmetrically, creating a gradient between the illuminated and shaded sides (or upper and lower sides), leading to organ bending.

Polar transport is what transforms auxin from an ordinary hormone into the architect of plant form. Without it, there would be no upright stem, no directed root growth, no leaf mosaic—everything we consider the "normal" appearance of a plant (Schopfer & Brennicke, 2016; Taiz et al., 2023; Connor et al., 2011).

Main takeaway of this section:

Auxin moves in only one direction—from tip to base—thanks to polarly localised efflux carriers (PIN proteins) in cell membranes. This creates a stable concentration gradient that serves as a coordinate system for the entire plant. Polar transport is the "skeleton" of auxin regulation, upon which the entire architecture of the plant body rests. In the next section, we will see how this mechanism manifests at the whole-plant level—in the phenomenon of apical dominance.

Apical dominance—hierarchy in the plant world

If you have ever observed the growth of a tree or shrub, you have noticed that most plants have one main, central axis (trunk or stem) that grows vertically upward and dominates over lateral branches. Lateral shoots either do not develop at all or grow much more slowly and subordinately. This phenomenon is called apical dominance—the suppression of growth of lateral (axillary) buds by the apical bud (Kuznetsov, 2006; Medvedev, 2012; Schopfer & Brennicke, 2016).

Why does this happen? The answer lies in the polar transport of auxin discussed in the previous section. The apical bud is the main source of auxin in the young shoot. It synthesises IAA and sends it down the stem (basipetally). As it moves downward, the auxin concentration gradually decreases. It is this downward auxin flow that serves as the signal suppressing the activation and growth of axillary buds (Hopkins & Hüner, 2009; Taiz et al., 2023; Medvedev, 2012).

This phenomenon was first demonstrated in classic experiments back in the 1930s. If the shoot tip is removed (decapitation), after some time the lateral buds begin to grow actively—the plant loses its "main apex" and becomes bushier. If an auxin-containing paste (IAA) is applied to the cut surface, the suppression of lateral buds is restored—auxin completely replaces the removed tip (Schopfer & Brennicke, 2016; Hopkins & Hüner, 2009). This is a direct and convincing experiment showing that it is auxin, not any other factor, that is responsible for apical dominance.

How does the suppression mechanism work?

For a long time, it was believed that auxin directly inhibits lateral bud growth by acting on them "directly." However, modern research shows that the mechanism is more complex. It turns out that auxin itself is not a direct inhibitor of the axillary bud. Moreover, applying auxin directly to a lateral bud often does not cause suppression (and may even stimulate growth). So what is actually happening?

The key element is a signalling pathway involving another hormone—cytokinin. Axillary buds require cytokinin supply from the roots to become active. Auxin coming from the shoot tip suppresses cytokinin synthesis in the root system or blocks its transport to the lateral buds (Medvedev, 2012; Taiz et al., 2023). In addition, auxin activates the synthesis of strigolactones (a recently discovered group of phytohormones), which also suppress lateral bud growth, acting together with auxin (Taiz et al., 2023; Schopfer & Brennicke, 2016). Thus, auxin does not act directly but through a whole network of interactions with other hormones.

It is important to understand: apical dominance is not merely "suppression" but redistribution of resources in favour of the main axis. Auxin has a so-called attracting effect—it draws nutrients, water, and organic and mineral compounds towards itself. Where auxin concentration is higher, assimilates are directed there. The shoot tip becomes a powerful "pump" that funnels resources to itself, leaving lateral buds in a state of "starvation" (Medvedev, 2012; Kuznetsov, 2006). This is a vivid example of how one hormone, through a concentration gradient, determines not only the form but also the physiology of the whole plant.

What happens after the tip is removed?

Removal of the apical bud is a technique humans have used since ancient times. In horticulture, it is called pinching or topping. When we remove the tip, the downward auxin flow ceases. The lateral buds (which had been inhibited) receive "permission" to grow. The closer a bud is to the cut site (i.e., the higher up the stem), the faster it awakens—because it had received more auxin in the past and was more strongly inhibited, but now it recovers more quickly (Schopfer & Brennicke, 2016).

Interestingly, it is usually the nearest lateral bud to the cut that begins to grow, becoming the new "main apex." This process is called replacement of the apical bud. If we want a branched, bushy plant, we can repeatedly pinch the growing lateral shoots, causing them to branch further. This is the basis for shaping fruit tree crowns, pinching houseplants, suckering tomatoes, and many other agronomic practices (Medvedev, 2012; Kuznetsov, 2006).

Why is this important for understanding growth?

Apical dominance is one of the most illustrative examples of how a single hormone creates the entire architecture of the plant. It demonstrates:

1. Organ hierarchy: there is a main shoot and subordinate ones—this is not accidental but a tightly regulated process.

2. Integration of different hormones: auxin works in concert with cytokinins, strigolactones, and other regulators.

3. Practical significance: understanding this mechanism allows us to manipulate plant shape, stimulate branching, or maintain a compact crown.

In nature, apical dominance gives the plant a competitive advantage—rapid upward growth allows it to outcompete neighbours and capture more light. Thus, it is not merely an "internal arrangement" but also an important evolutionary adaptation mechanism (Connor et al., 2011; Schopfer & Brennicke, 2016).

Connection with polar transport

Apical dominance is a direct consequence of polar auxin transport, which we discussed in the second section. Without directed movement from tip to base, there would be no concentration gradient, and without a gradient, there would be no suppression of lateral buds. It is polar transport that creates the very "axis of hierarchy" that determines which shoot will be main and which secondary.

This vividly illustrates the key idea of the entire course: plant form is not merely a genetic programme but also the result of physicochemical processes that can be understood and even manipulated. In the following sections, we will see how the same principles (gradient and redistribution of auxin) underlie tropisms—the plant's responses to light and gravity.

Main takeaway of this section:

The plant has one main apex because of apical dominance—the suppression of lateral buds by the auxin flow from the apical bud. This is not direct inhibition but a complex regulatory network involving cytokinins and strigolactones, as well as nutrient redistribution. Removing the tip releases this suppression, allowing us to manipulate plant shape in agronomy. Apical dominance is a clear example of how polar auxin transport creates the hierarchical structure of the plant.

Introduction: the problem of orientation

Plants lead a sessile lifestyle. They cannot move to a more illuminated place, flee from drought, or bypass an obstacle. However, this does not mean they are passive. On the contrary, plants possess an amazing ability to orient themselves in space—directing their organs to optimally use environmental resources. Roots must grow downward, into the soil depth, to obtain water and minerals. Shoots must grow upward, towards light, to photosynthesise efficiently. How does a plant "know" which way is up and which way is down? And how does it "decide" which way to bend?

The answers lie in tropisms—growth movements of organs in response to a unilaterally acting external stimulus (Kuznetsov, 2006; Medvedev, 2012). If an organ bends towards the stimulus source, it is a positive tropism (e.g., a shoot bends towards light). If it bends away, it is a negative tropism (e.g., a root grows downward, away from light). All tropisms are based on the same mechanism: asymmetric redistribution of auxin in response to a directed external signal (Hopkins & Hüner, 2009; Schopfer & Brennicke, 2016; Taiz et al., 2023). This redistribution creates a concentration gradient of the hormone between the opposite sides of the organ, leading to unequal growth rates and, consequently, bending.

Phototropism: how the plant reaches for the light

We have already touched upon phototropism in the historical introduction when discussing the experiments of Darwin and Went. Now let us examine its mechanism in more detail. Phototropism is the bending of plant organs towards a light source (positive in shoots, negative in many roots). The key stages of this process (Hopkins & Hüner, 2009; Schopfer & Brennicke, 2016; Taiz et al., 2023):

1. Light perception. Light is perceived by the tip of the shoot or coleoptile. The main photoreceptors are photopigments (e.g., phototropin for blue light, as well as phytochromes and cryptochromes). The primary response to the light stimulus occurs in the tip.

2. Auxin redistribution. In response to unilateral illumination, auxin (IAA) begins to move laterally—from the illuminated side to the shaded side. This process is mediated by the redistribution of PIN proteins (efflux carriers) in the plasma membrane of tip cells (Medvedev, 2012; Taiz et al., 2023). As a result, the auxin concentration on the shaded side becomes higher than on the illuminated side.

3. Differential growth. Since auxin at optimal concentrations stimulates cell elongation in shoots, cells on the shaded side grow faster than those on the illuminated side. This causes the organ to bend towards the light source (Hopkins & Hüner, 2009; Schopfer & Brennicke, 2016).

This mechanism has been confirmed by classic experiments using labelled auxin and polar transport inhibitors. For example, applying an auxin transport inhibitor (e.g., NPA—N‑1‑naphthylphthalamic acid) to the coleoptile tip blocks the phototropic response (Taiz et al., 2023). Thus, phototropism is not just a "reaction to light" but a complex process in which the light signal is translated into a chemical one—a change in auxin distribution.

Special case: roots and phototropism. Unlike shoots, many roots exhibit negative phototropism—they grow away from light. This is explained by the fact that root cells have a different sensitivity to auxin: concentrations that stimulate shoot growth actually inhibit root growth. Therefore, the light-induced redistribution of auxin in the root causes the illuminated side, which receives less auxin, to grow faster, while the shaded side (with more auxin) grows slower, causing the root to bend away from the light (Taiz et al., 2023; Lambers & Oliveira, 2019).

Gravitropism: how the plant senses gravity

No less remarkable is the plant's ability to determine the direction of gravity. Gravitropism (geotropism) is growth in response to gravity. Shoots exhibit negative gravitropism (grow upward), while roots exhibit positive gravitropism (grow downward) (Kuznetsov, 2006; Medvedev, 2012; Schopfer & Brennicke, 2016). How does it work?

1. Gravity perception. Perception occurs in specialised cells—statocytes. In roots, these are cells of the root cap (columella); in shoots, they are cells of the starch sheath (endodermis) around the vascular bundles (Taiz et al., 2023; Schopfer & Brennicke, 2016). Inside these cells are dense statoliths—amyloplasts containing large starch grains. In a vertically growing organ, statoliths are evenly distributed. But as soon as the organ deviates from the vertical, statoliths sediment onto the lower cell wall under gravity (Hopkins & Hüner, 2009; Medvedev, 2012). This mechanical displacement serves as the signal that triggers a cascade of events.

2. Auxin redistribution. The displacement of statoliths in root cap or starch sheath cells causes changes in PIN protein activity—they are redistributed on the lower side of the cells, and auxin begins to be actively transported to the lower side of the organ (Taiz et al., 2023). As a result, a gradient is established: auxin concentration on the lower side becomes higher than on the upper side.

3. Differential growth. Here, the difference in auxin sensitivity between root and shoot becomes evident (Schopfer & Brennicke, 2016; Taiz et al., 2023):

• In the shoot (stem, coleoptile), the elevated auxin concentration on the lower side stimulates cell elongation. Therefore, the lower side grows faster than the upper side, and the shoot bends upward (negative gravitropism).
• In the root, the elevated auxin concentration on the lower side, conversely, inhibits cell elongation (since roots are sensitive to high IAA concentrations). The upper side (with less auxin) grows faster, and the root bends downward (positive gravitropism) (Lambers & Oliveira, 2019; Medvedev, 2012).

This principle was brilliantly confirmed in experiments with root cap removal: decapitated roots lose gravitropic response, but it is restored if an auxin-containing agar block is placed on the cut surface (Taiz et al., 2023; Schopfer & Brennicke, 2016).

Why do roots and shoots respond to auxin differently?

This is one of the most important questions, showing that the effect of a hormone depends on the tissue, not only on its concentration. Shoots and roots have different sets of receptors and signalling pathways. In shoots, auxin activates proton pumps, acidifies the cell wall, activates expansins—and cells elongate (Hopkins & Hüner, 2009). In roots, the same concentration may activate ethylene synthesis, which inhibits elongation, or directly affect gene expression that suppresses growth (Medvedev, 2012; Taiz et al., 2023). It is this tissue specificity that makes auxin a universal regulator capable of controlling opposite processes in different parts of the plant.

Other tropisms: a brief overview

Beyond light and gravity, plants respond to many other directional stimuli (Kuznetsov, 2006; Medvedev, 2012):

• Thigmotropism – bending in response to touch (tendrils of climbing plants twine around supports).
• Chemotropism – growth towards a chemical gradient (roots grow towards nutrient sources; pollen tubes grow towards the ovule).
• Hydrotropism – root growth towards moister soil patches.
• Aerotropism – root growth towards better aeration.

All these responses, if based on differential growth, are also mediated by auxin (or other hormone) redistribution. Thus, we see a general principle: any directed external stimulus, perceived by certain cells, is translated into an asymmetric distribution of auxin, which in turn causes unequal growth of the opposite sides of the organ (Schopfer & Brennicke, 2016).

What does this system give the plant?

The ability to perform tropisms is a key survival factor for plants. It allows them to:

• Direct roots into the soil depth, towards water and minerals (positive gravitropism).
• Direct shoots upward, towards light (negative gravitropism and positive phototropism).
• Avoid obstacles (thigmotropism) and find nutrient sources (chemotropism).
• Ensure pollination (chemotropism of pollen tubes).

In agronomy, knowledge of these mechanisms allows us to manage the growth of cultivated plants: for example, using light regimes to form compact canopies, creating optimal conditions for root systems, and applying growth regulators to modify tropic responses (Medvedev, 2012; Kuznetsov, 2006).

Connection with previous sections

Now we can see how all the pieces—polar transport, concentration gradients, apical dominance, and tropisms—fit into a single picture. Auxin, moving polarly, establishes the basic axis of the plant. Apical dominance determines the shoot hierarchy. The redistribution of auxin under light and gravity allows the plant to orient itself in space and flexibly respond to environmental changes. All of this is manifestations of the same hormonal regulatory system.

Main takeaway of this section:

The plant determines which way is up and which is down through tropisms—growth movements in response to directed external stimuli. All tropisms are based on asymmetric auxin redistribution: under light or gravity, auxin shifts to one side of the organ, creating a gradient that causes differential growth. Shoots and roots respond to this gradient differently due to tissue-specific auxin sensitivity, ensuring bending in the appropriate direction. Thus, auxin is a universal mediator of plant spatial orientation.

Introduction: roots—the basis of survival

We have already discussed how auxin determines the shape of the above-ground part of the plant—from the main shoot to spatial orientation. But the plant has another, equally important part that remains hidden from our eyes: the root system. Roots are the "invisible front" of the plant. They obtain water and nutrients, anchor the plant in the soil, and engage in complex interactions with microorganisms. Without a well-developed root system, even the most vigorous shoot cannot survive.

How does the plant "decide" when and where to form new roots? The answer, as you might have guessed, is again linked to auxin. But here there is an important twist: auxin, which in the shoot stimulates cell elongation, triggers a completely different process in the root—the formation of new root primordia and their subsequent development (Medvedev, 2012; Taiz et al., 2023). This is yet another example of the remarkable plasticity of one hormone’s action depending on the tissue context.

In this section, we will consider two main types of new root formation stimulated by auxin:

1. Adventitious (accessory) root formation – root formation from unusual places: stems, leaves, even flower stalks. This is the process used in plant propagation by cuttings.

2. Formation of lateral roots – branching of the existing root system.

Adventitious root formation: how a cutting becomes a plant

Imagine: you cut a twig from a plant, place it in water, and after some time white roots appear at the lower end. This is adventitious root formation. It has enormous practical significance: it is the basis for vegetative propagation by cuttings, widely used in horticulture, forestry, and ornamental plant cultivation (Kuznetsov, 2006; Medvedev, 2012; Marschner, 2012).

What happens at the physiological level? When we detach a cutting from the mother plant, we interrupt the auxin supply from the tip. However, some auxin remains in the cutting tissues, and moreover, stem cells begin to synthesise it locally. Auxin accumulates in the basal (lower) part of the cutting—precisely where roots will later form (Taiz et al., 2023; Schopfer & Brennicke, 2016; Medvedev, 2012).

But auxin alone is not enough. Successful root formation requires a high local concentration of auxin in the future root zone, exceeding the threshold needed for shoot growth stimulation. This is why synthetic auxins are often used in cutting practice (e.g., indolebutyric acid—IBA, or naphthaleneacetic acid—NAA): they are more stable against oxidation and provide a more consistent concentration (Medvedev, 2012; Marschner, 2012; Taiz et al., 2023).

How does this mechanism work at the cellular level? Auxin induces the expression of genes in stem cells that trigger a dedifferentiation programme—parenchyma cells lose their specific characteristics and acquire the ability to divide. They then begin to divide and organise into a root primordium—a structure that will subsequently give rise to a root (Schopfer & Brennicke, 2016; Taiz et al., 2023). It is important to emphasise: auxin not only gives the "signal to start" but also determines the spatial organisation of the new organ. The root primordium forms precisely where auxin concentration reaches a critical level.

Interestingly, adventitious roots can form not only on stems but also on leaves (e.g., in some succulents or Saintpaulias) and even on flower stalks. Wherever there is tissue capable of dedifferentiation and where auxin reaches the necessary concentration, new roots may emerge (Medvedev, 2012).

Lateral roots: branching of the root system

Adventitious roots are an "emergency" route that the plant uses when standard branching pathways are disrupted (e.g., when the primary root is damaged or during cuttings). Normally, the root system branches through the formation of lateral roots from the pericycle (the tissue lying just inside the endodermis) (Taiz et al., 2023; Lambers & Oliveira, 2019).

This process is also regulated by auxin, but with an important difference. Lateral roots are formed not in response to a high auxin concentration, but in response to its gradient. In a growing root, auxin is transported from the tip to the base (apical-basal flow) and creates a characteristic gradient: near the root tip, the concentration is high (but not maximal), and closer to the differentiation zone, it decreases. Certain pericycle cells in this zone "sense" this gradient, and when auxin reaches a certain level, they begin to divide, giving rise to a lateral root (Taiz et al., 2023; Schopfer & Brennicke, 2016).

Moreover, modern research shows that a local increase in auxin concentration in pericycle cells occurs thanks to redistribution of PIN proteins. In the zone where a lateral root is to be initiated, auxin begins to be transported laterally (sideways), creating a local maximum that serves as a signal for cell division (Medvedev, 2012; Taiz et al., 2023). This process is somewhat reminiscent of leaf primordium formation at the shoot apical meristem—in both cases, local auxin gradients determine where a new organ will arise.

Why does auxin "switch on" different programmes?

In the shoot, auxin at high concentrations stimulates cell elongation. In the stem during cutting, it triggers dedifferentiation and cell division leading to root formation. In the root, it (depending on concentration) may stimulate or inhibit growth and also initiate lateral root primordia. How can the same hormone elicit such different responses?

The answer lies in tissue-specific receptors and signalling cascades. Different cell types contain different sets of proteins that perceive auxin and transmit its signal. In shoot cells, high auxin concentrations activate proton pumps and expansins (wall-loosening proteins). In stem cells, auxin activates genes responsible for dedifferentiation and division (so-called primary response genes, e.g., the GH3 family). In the root pericycle, the same auxin, through its receptors (TIR1/AFB proteins), triggers the degradation of AUX/IAA repressor proteins, releasing ARF transcription factors that activate genes necessary for lateral root formation (Medvedev, 2012; Taiz et al., 2023; Hopkins & Hüner, 2009). Thus, the outcome of auxin action is determined not only by its concentration but also by the cell type.

Practical significance for agronomy

Understanding how auxin controls root formation has enormous practical significance (Medvedev, 2012; Marschner, 2012; Kuznetsov, 2006):

1. Cutting propagation. Treating cuttings with synthetic auxins (IBA, NAA) allows the rooting of species that normally root poorly (e.g., many woody species, conifers, roses).

2. Regulation of the root system. Understanding that auxin stimulates lateral root formation allows the development of treatment regimes that enhance root system branching, increasing its absorptive capacity.

3. Stress management. Under drought or mineral deficiency, plants often enhance root formation to increase resource uptake. Knowledge of auxin mechanisms helps breeders create varieties with more robust root systems.

4. Microbiological interactions. Auxin released by soil microorganisms (e.g., nitrogen-fixing bacteria) can stimulate root formation, improving plant growth. This is actively used in biotechnology (Marschner, 2012).

Connection with previous sections

Root formation is another manifestation of how auxin creates the architecture of the plant, completing the picture. We began with auxin determining the main shoot (polarity), then why there is one shoot (apical dominance), then how it orients itself in space (tropisms). Now we see that the same system regulates the underground part: it determines when and where new roots will arise and how the root system will branch.

Thus, auxin is a universal architect of the whole plant—both above-ground and below-ground. Its action is based on three principles: polar transport (creating gradients), asymmetric redistribution (in response to stimuli), and tissue-specific sensitivity (determining the type of response). It is thanks to these principles that one hormone can govern the entire complexity of plant form and function.

Main takeaway of this section:

Auxin triggers the formation of new roots through two pathways: adventitious root formation (from stems or leaves, e.g., during cutting propagation) and lateral root formation (branching of the root system). In the first case, a high auxin concentration causes dedifferentiation of cells and formation of a root primordium; in the second, local auxin gradients in the pericycle initiate cell division, giving rise to a lateral root. The same hormone produces different outcomes in different tissues due to distinct receptors and signalling pathways. This demonstrates the universality of auxin as a regulator of the architecture of the entire plant—both its aerial and underground parts.